Oil well pumps operated by fluid pressure sources located at the earth's surface are known and examples may be found in U.S. Pat. Nos. 2,338,903 and 4,490,095 by way of example. As is well known in this art, the problems of operating pumps in the hostile environment of oil wells are many. The fluids encountered are corrosive in addition to being highly abrasive and consequently it is desirable to have pumps with as few working parts as possible and ones which expose as few working parts to as little of the hostile environment as possible. Replacement or repair of such pumps is both time consuming and expensive because it involves removing the pump from great depth to the earth's surface.